This invention relates to the recovery of gold from ores by thiosulfate leaching and, more particularly, to an ion exchange process for the recovery of gold from a thiosulfate leach solution or slurry.
Gold is typically recovered using conventional cyanidation technology. The pH is adjusted to between 10 and 11 with lime, and cyanide is added to solubilize the gold. Oxygen is dispersed through the slurry by agitation, and gold dissolves by the following reaction:
4Au+O2+8CNxe2x88x92+H2Oxe2x86x924Au(CN)2xe2x88x92+4Oxe2x88x92
In modern cyanidation circuits, the dissolved gold is typically adsorbed onto particles of activated carbon, either during the cyanide leach itself by carbon-in-leach (CIL) or following the leach by carbon-in-pulp (CIP). An alternate method of recovering gold from cyanide leach solutions is through zinc cementation and variations of the Merrill-Crowe process.
In addition to the locking of gold particles in sulfide minerals, a problem which must be addressed in the treatment of some ores is preg robbing. In carbonaceous ores, preg robbing occurs as active carbon indigenous to the ore has the ability to rob gold from the cyanide bearing leach solution, reducing recovery. Pressure oxidation can partially deactivate the indigenous carbon, but by itself is not sufficient for highly preg-robbing ores. To further reduce preg-robbing problems, blanking agents such as kerosene or sodium lauryl sulfate have been used to further deactivate carbon in the ore. Carbon-in-leach has been successful for mildly preg-robbing ores, as the activated carbon added to the slurry possesses adsorption kinetic characteristics superior to those of the indigenous carbon, allowing the gold to load onto the added carbon as soon as it is leached, before it can load onto the carbon in the ore. Carbon-in-leach alone, however, has not been completely successful in treating highly preg-robbing ores.
An additional problem in recovering gold from highly carbonaceous ores is that a significant quantity of the gold may have been adsorbed onto carbon during formation of the mineral deposit. This gold will only become available to a lixiviant which can remove it from the carbon. The use of a cyanide lixiviant alone has not been entirely successful in leaching gold locked in carbonaceous material.
Thiosulfate leaching of gold is a potentially attractive alternative to the conventional cyanidation process for at least three types of gold ore feed material. First, in gold ores that contain organic carbonaceous material, gold recovery by thiosulfate leaching is usually significantly higher because the gold thiosulfate complex is quite insensitive to preg robbing. Secondly, gold/copper ores are frequently not well suited to the cyanidation process owing to higher cyanide consumption by the copper mineralization in the ore, which leads to unacceptably high operating costs. Thiosulfate does not react as readily with copper minerals, and the lower reagent cost and consumption of thiosulfate compared to cyanide leads to potentially lower operating costs in these situations. Finally, there are certain gold ore bodies that cannot be treated by the conventional cyanidation process because they are located in environmentally sensitive areas. Thiosulfate leaching reduces the impact on the environment, as the chemicals used in this process are already used as fertilizers in the agriculture industry.
The thiosulfate leach process has been proven to be a technically viable process, with many aspects of the process disclosed in publications and patents. For example, Berezowsky et al., U.S. Pat. No. 4,070,182, disclose a process to leach gold from copper-bearing sulphidic material with ammonium thiosulfate, followed by cementation of the gold on zinc dust. Kerley Jr., U.S. Pat. Nos. 4,269,622 and 4,369,061, disclose using an ammonium thiosulfate leach solution containing copper to leach gold and silver from ores containing manganese. Perez et al., U.S. Pat. No. 4,654,078, disclose leaching gold and silver with a copper-ammonium thiosulfate lixiviant to produce a pregnant leach solution. Gold and silver are then precipitated onto a copper cement added to the pregnant leach solutions. Wan et al., U.S. Pat. No. 5,354,359, disclose leaching gold from preg robbing ores with a thiosulfate lixiviant followed by cementation or precipitation of the leached precious metal values. PCT application WO 91/11539 discloses recovering gold from a gold-loaded thiosulfate solution by adding cyanide to form a gold cyanide complex followed by adsorbing the gold cyanide complex onto a carbon or resin adsorbent. Thomas et al., U.S. Pat. Nos. 5,536,297 and 5,785,736, disclose a process for treating a refractory sulphidic and carbonaceous ore by pressure oxidation followed by thiosulfate leaching and adsorption of the gold thiosulfate complex on an ion exchange resin.
The processes that have been disclosed to extract gold from the thiosulfate leach liquors include cementation on zinc (Berezowsky, et al.) or copper (Perez et al., Wan et al.), conversion of gold thiosulfate to gold cyanide, followed by adsorption on activated carbon (PCT Application WO 91/11539), and adsorption on ion exchange resin (Thomas, et al). These processes are very efficient metallurgically, however, they each have limitations. For example, the cementation processes require that the leach slurry first be processed by filtration or counter-current decantation to separate the leach solution from the leached solids. This process is expensive and can result in appreciable gold losses due to re-precipitation or entrainment in the leached solids. The process disclosed in PCT Application WO 91/11539 also cannot be used to treat carbonaceous preg robbing feed materials without solid/liquid separation prior to final gold recovery.
The process disclosed by Thomas et al., can be used to recover gold thiosulfate from solutions or pulp without solid/liquid separation, and recovers gold efficiently from carbonaceous, preg robbing ores. However, thiocyanate salts are quite expensive, and significant losses of thiocyanate to the tailings can have a material effect on the overall economics of the process, as well as potentially creating environmental problems. In addition, because of the great affinity of the thiocyanate ion for ion exchange resins, it is important to regenerate the eluted resin, to displace thiocyanate, prior to recycling the resin to adsorption. Regeneration with sulfuric acid is effective, but this process increases operating costs and reduces resin life.
The process disclosed herein then, reveals an alternative method for recovering gold from thiosulfate leach solutions that can recover gold from ores, concentrates or other feed materials more economically than the conventional cyanidation process.
Among the several objects of the present invention, therefore, is the provision of a process for recovering gold from comminuted ores, concentrates or other feed materials wherein gold has been leached to form a gold-bearing thiosulfate leach solution or slurry; the provision of such a process wherein gold is recovered after contacting a gold-bearing thiosulfate leach solution or slurry with an ion exchange resin; and the provision of such a process wherein gold is recovered after contacting a gold-loaded ion exchange resin with polythionate ions. Another object is the provision of a process for manufacturing a polythionate ion solution for use in eluting gold from an ion exchange resin. A further object of the present invention is the provision of a process for the recovery of gold from polythionate eluate solutions.
Generally, therefore, the present invention is directed to a process for recovering gold from comminuted ore. The process comprises preparing an aqueous gold-bearing slurry comprising a solid ore residue, a thiosulfate lixiviant and an ion exchange resin. Gold is then transferred from the slurry to the ion exchange resin. The gold is then eluted from the resin by contacting the resin with polythionate ions to form a gold-bearing eluate from which gold is then recovered.
The present invention is further directed to a process for recovering gold from a thiosulfate leach solution containing a gold-bearing thiosulfate lixiviant. The process comprises contacting the leach solution with an ion exchange resin having an affinity for gold to adsorb gold on the resin. The resin is then contacted with polythionate ions to elute the gold, producing a gold bearing eluate from which the gold is recovered.
The present invention is still further directed to a process for manufacturing a polythionate ion solution for use in eluting gold from an ion exchange resin. The process comprises oxidizing thiosulfate to polythionate by contacting a thiosulfate solution with about 75 to about 100 percent of a stoichiometrically equivalent amount of an oxidant.
The present invention is still further directed to a process for recovering gold from a gold-bearing eluate resulting from the polythionate elution of a gold-loaded ion exchange resin. The process comprises contacting the eluate with a solution of sulfide ions to form an insoluble gold sulfide species.
These and other objects, features and advantages of the invention will become apparent from the following detailed description.